This invention relates to an electron beam illumination apparatus used in the lithography process for the manufacture of semiconductor devices, and to an electron beam exposure system having this illumination apparatus.
An optical stepper having a high productivity has been used in the mass-production stage of semiconductor memory device manufacture. In the production of memory devices such as 1 G and 4 G DRAMs and beyond having a line width of less than 0.2 xcexcm, however, the electron beam exposure method, which features a high resolution, is expected to be one of the exposure techniques that will replace optical exposure.
The mainstays of conventional electron beam exposure have been the single-beam Gaussian method and the variable shaping method. Because these electron beam exposure methods exhibit low productivity, they have been used in mask lithography, research and development of very large scale integration and exposure of ASIC devices manufactured in low volumes.
Thus, the challenge involved in adapting electron beam exposure methods for mass production is how to raise the productivity of these methods. One proposed solution is partial batch transfer. An exposure apparatus of the variable shaping or partial batch transfer type is shown in FIG. 8A. This apparatus includes an illumination apparatus 21 constituted by an electron gun 20 having an electron source 7b, a Wehnelt electrode 8, a ground electrode 9 and an aperture 10a, a deflector 13, a first illumination lens 11a, a second illumination lens 11b and an aperture 10b (or electron beam mask 15); and a projection lens system 22 having a projection lens 12 for projecting an electron beam, which has passed through the electron beam mask 15, onto a wafer 16 on a stage 17 via a deflector 14. An exposure system of the partial batch transfer type is particularly good method at raising exposure productivity because it is capable of reducing the number of exposure shots by dividing the repetitive portions of memory circuit patterns into cells of several microns.
Assuring exposure line-width precision at the same time as the productivity of an exposure system is an important practical factor in the exposure system. It is required that uniformity of the intensity of irradiation in an exposure area be made less than 0.5% over the entire exposure area. In order to obtain an irradiating electron beam having a high degree of uniformity, an electron beam of good characteristics in an area having an aperture angle of several milliradians is extracted as the irradiating beam, through screening by the aperture 10a, from an electron beam of tens of milliradians emitted from the electron gun (see FIG. 8B).
A method of enlarging the exposure area has recently been proposed in order to provide a further improvement in productivity. For example, SCALPEL, which is an electron beam transfer method using an electron beam scattering mask [S. D. Berger et al., xe2x80x9cProjection electron beam lithography: A new approachxe2x80x9d, J. Vac. Sci. Technology B9, 2996, (1991)], is one method of high-speed electron beam exposure. With this method the exposure area is 2500 times larger than in the case of the conventional variable shaping method or cell exposure method and, as a result, it is possible to reduce the effects of electron interaction caused by the Coulomb effect. This makes it possible to perform exposure upon raising the irradiating electron beam current by one order of magnitude over the conventional exposure method. A high productivity is expected as a result. However, this electron beam exposure method requires a highly uniform irradiation intensity over a wide exposure area. In addition, an increase in the irradiation current is accompanied by an increase in the amount of electron beam screened by the apertures in the electron gun and illumination column after the electron beam is generated by the electron source.
With an electron beam mask projection apparatus that uses an arcuate beam (see the specification of Japanese Patent Application Laid-Open No. 10-135102), the width of the exposure area is several millimeters, which is about six times greater than in the projection exposure method (SCALPEL) mentioned above. As a result, high-speed exposure is possible by scanning exposure of the mask and wafer. In order to illuminate a wide area uniformly, however, part of the peripheral beam of the electron beam produced by the electron gun is used as an irradiating beam of the exposure in the form of a ring-shaped or arcuate beam. In particular, when use is made of an electron beam emitted from an electron gun uniformly, almost all of the emitted electron beam in the generated electron beam must be screened by the apertures in the electron gun and illumination column, and a problem that arises is that the utilized efficiency of the emitted electron beam is reduced to about 1/1000.
With conventional exposure methods, the irradiating electron beam is small in quantity and no particular problems arise even if the electron current is screened by the aperture electrodes in the electron gun and illumination column. However, with the above-described exposure method having a high productivity, the current of the irradiating electron beam on the wafer is tens of microamps, which means that exposure is performed using a current that is greater by more than one order of magnitude in comparison with the conventional exposure methods. The electron current screened by the aperture electrodes takes a large value of several milliamps.
As a consequence, with the conventional method in which the emitted electron beam is screened by the beam screening electrodes along the beam path, one problem that arises is that these electrodes rise in temperature and may fuse. Another disadvantage is that a rise in temperature within the illumination column and a charge-up phenomenon within the column caused by scattering of screened electrons can lead to a decline in the positioning precision of the irradiating electron beam, which is an important factor in exposure performance. This makes it difficult to improve throughput by increasing irradiation current. Furthermore, since the load on the power supply of the electron generating device is larger than that in the conventional electron generating device, obtaining a highly stable, inexpensive power supply is difficult.
Accordingly, an object of the present invention is to provide an electron beam illumination apparatus for eliminating the problems of the prior art, namely a decline in beam positioning precision, and for improving on efficiency with which an emitted electron beam is utilized and throughput, and reducing equipment cost, as well as an electron beam exposure system having this illumination apparatus.
According to the present invention, the foregoing object is attained by providing an electron beam illumination apparatus having an electron gun which generates electrons from an electron emission surface and accelerates the electrons for irradiating an irradiation area with an electron beam, the electron gun having a thermal-electron source and the electron emission surface of the electron source having an effective irradiation area an exposure irradiation area and a restricted irradiation area the surface of the effective irradiation area and the surface of the restricted irradiation area consisting of materials having different electron emission efficiencies.
The surface materials of the electron emission surface are characterized in that the difference between the electron emission efficiencies at the electron emission surface is a difference between work functions, with the work function of the restricted irradiation area being greater than that of the effective irradiation area by 1.0 eV or more. Further, the effective irradiation area consists of lanthanum boride having a high electron emission efficiency, and the restricted irradiation area comprises carbon, or a material containing carbon, that will not react with the material of the effective irradiation area at high temperatures. Furthermore, the effective irradiation area and the restricted irradiation area of the electron emission surface both comprise metals exhibiting a high melting point, the material of the effective irradiation area comprising a monoatomic layer of cathodic material, and the restricted irradiation area comprising tungsten, platinum or palladium.
A method of forming an electron emission layer having such a restricted irradiation area includes forming a surface layer of the restricted irradiation area by vapor deposition, ion-beam sputtering, ion plating or ion injection means, thereby forming the surface of the restricted irradiation area.
Further, the electron emission surface of the thermal-electron source of the electron beam illumination apparatus is flat in shape and has the restricted irradiation area disposed on the periphery of the surface. This arrangement is ideal for an electron beam exposure system having a planar exposure area.
Further, in an exposure scheme in which the exposure area is arcuate in shape, it is preferred that the electron emission surface have a flat shape and that the effective irradiation area be formed in the shape of a ring or partial arc on the surface. Alternatively, the electron emission surface is concave in shape, the effective irradiation area is provided in the shape of a ring or arc on the outermost surface, and the restricted irradiation area is provided on the periphery of this effective irradiation area. Alternatively, the electron emission surface is a concave or convex curved surface, the effective irradiation area is provided in the shape of a ring or arc on the surface, and the restricted irradiation area is provided on the periphery of this effective irradiation area.
In an exposure scheme in which the exposure area has different aspect ratios, it is preferred that the electron emission surface have a flat shape and that the effective irradiation area be formed on the surface in a straight line.
Furthermore, in an exposure scheme in which multiple exposure areas are exposed simultaneously by making use of multiple electron beams, it is preferred that the effective irradiation areas be dispersed over the electron emission surface.
Thus, as described above, the electron emission surface of a thermal-electron source of an electron gun is provided with an effective irradiation area and a restricted irradiation area the electron emission efficiencies of which differ. The effective irradiation area is adopted as an effective electron emission area that participates in exposure. The restricted irradiation area is adopted as an area which does not participate directly in exposure and which emits an electron beam that, if were not restricted, would be screened by the aperture electrodes in the electron gun or illumination column. As a result, the irradiation utilization efficiency of the emitted electrons can be improved. Furthermore, since the quantity of electron beams that irradiate the aperture electrodes in the illumination column is greatly reduced, the problems of the rise in temperature within the column, column charge-up and fusing of aperture electrodes caused by irradiation with the electron beam are solved. In addition, a decline in beam resolution due to electron interaction can be prevented by extracting only the necessary electron beams within the illumination column.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.